Parturition is a physiological process that occurs when pregnant females are in labor. It is characterized by increasingly frequent uterine contractions and cervical effacement, which ultimately leads to delivery of offspring. Parturition is a complex physiological and molecular biological process that has evolved differently in different species due to each species' unique environmental and temporal niches. Most mammals have adapted to selective pressures, such as the availability of food and prevalence of predators, by developing either a diurnal or nocturnal activity phase. Pregnant females have adapted to deliver their offspring in their den or home camp rather than in the field, which enhances their safety, security, and birth success.
The selective advantage for entering parturition during the daytime or nighttime phases is reflected in the differential timing of this event among many species of nocturnal rodents and diurnal mammals, such as sheep and primates. Rats, for example, enter parturition predominantly during the day time, even when the light-dark cycles are reversed. [1-3] Similarly, golden hamsters develop strong uterine contractions and deliver their young during daytime hours. [4] Humans, on the other hand, tend to enter labor during the late nighttime and early morning hours [5-9] with parturition typically following 12 to 24 hours thereafter, at least in nulliparous women. [7] The frequency of uterine contractions in preterm women at risk for spontaneous premature delivery increases significantly at night. [10]
In nonhuman primates, the late-term myometrial contractions and the sensitivity of the uterus to the contractile effects of oxytocin have been shown to be the highest early in the night phase. [11-13] In addition, the phasing of nocturnal parturition in nonhuman primates has been shown to also be shifted by reversal of the light/dark cycles [14], pointing to a light-sensitive clock mechanism underlying parturition. Since both humans and nonhuman primates show nocturnally peaking uterine contractions in late-term pregnancy [15-17], the intriguing question arises—what are the circadian signals that drive nocturnal uterine activity in late term human pregnancy?
Maintenance of appropriate circadian phase in peripheral tissues requires zeitgebers (entraining cues) that are coupled with the central circadian oscillator in the brain's suprachiasmatic nuclei (SCN) via neural pathways, rhythmic endocrine, and/or metabolic signals. For many peripheral clocks, such as the liver, heart, pancreas, and so on, autonomically driven neuroendocrine output cues such as melatonin and glucocorticoids are considered to play a key role. [18] Evidence continues to accumulate showing that these two hormones have significant effects on the endogenous circadian clockwork in a variety of peripheral tissues. [19-24]